Many chemical processes which include a reaction with chlorine or phosgene, such as the production of isocyanates or chlorination reactions of aromatic compounds, lead to formation of hydrogen chloride. Such formed hydrogen chloride can be converted back to chlorine by electrolysis, such as described in, for example, WO97/24320A1. In comparison to this type of energy-intensive method, the direct oxidation of hydrogen chloride with pure oxygen or an oxygen-containing gas in the presence of a heterogeneous catalyst (such as, for example, what is often referred to as the Deacon process) according to the following reaction:4HCl+O22Cl2+2H2Ooffers advantages in terms of energy consumption. Such a process is described, for example, in WO 04/014845, the entire contents of which are incorporated herein by reference.
In many processes which include a reaction with chlorine or phosgene, such as in particular phosgenation, a relatively large amount of carbon monoxide (CO) can be included in the resulting HCl containing waste gas as an impurity. In the generally widely used liquid phase phosgenation reactions, carbon monoxide in an amount from 0 to 3 vol. % can be found in the HCl waste gas from the phosgene scrubbing column. In state-of-the-art gaseous phase phosgenations, even higher CO amounts (up to more than 5%) can be expected, since in such methods preferably no condensation of phosgene, and therefore no associated large scale separation of the unreacted carbon monoxide, is carried out before the phosgenation.
In the conventional catalytic oxidation of hydrogen chloride with oxygen, a very wide range of catalysts can be employed, e.g., based on ruthenium, chromium, copper, etc. Such catalysts are described, for example, in DE1567788 A1, EP251731A2, EP936184A2, EP761593A1, EP711599A1 and DE10250131A1, the entire contents of each of which are herein incorporated by reference. Such catalysts can however at the same time act as oxidation catalysts for other components that may be present in a reaction stream, such as carbon monoxide or various organic compounds. The catalytic carbon monoxide oxidation to carbon dioxide is however extremely exothermic and can cause uncontrolled local temperature rises (hot spots) at the surface of heterogeneous catalysts, with the result that a deactivation of the catalyst with respect to the HCl oxidation may occur. For example, without cooling, the oxidation of 5% carbon monoxide in an inert gas (e.g., N2) at an inflow temperature of 250° C. (described operating temperatures in Deacon processes are generally 200°-450° C.) would result in a temperature rise of far above 200° C. One likely reason for the catalyst deactivation may be microstructural change of the catalyst surface, e.g., by sintering processes, on account of the formation of hot spots.
Furthermore the adsorption of carbon monoxide on the surface of the catalyst cannot be excluded. The formation of metal carbonyls may take place reversibly or irreversibly and may thus occur in direct competition to the desired HCl oxidation. Carbon monoxide can, at high temperatures, form very stable bonds with some elements, such as, e.g., osmium, rhenium, ruthenium (see, e.g., CHEM. REV. 103, 3707-3732, 2003), and may thereby inhibit the desired target reaction. A further disadvantage could arise due to the volatility of such metal carbonyls (see, e.g., CHEM. REV. 21, 3-38, 1937), whereby not inconsiderable amounts of catalyst are lost and in addition, depending on the application, an expensive and complicated purification step of the reaction product can be necessary.
Processes for the oxidation of hydrogen chloride with oxygen in which the carbon monoxide content of the gas that is used is adjusted in advance to less than 10 vol. % by palladium-catalysed combustion to form carbon dioxide, separation of the hydrogen chloride gas by distillation, or scrubbing of the gas with a solution of copper chloride to extend the lifetime of the catalyst, have been suggested.
In another known process a hydrogen chloride-containing waste gas is fed into an aqueous alkaline absorption system and the waste gas freed from hydrogen chloride and phosgene is sent to a combustion plant.
A disadvantage of the previously suggested processes for overcoming the aforementioned problems is the destruction of the a valuable carbon monoxide raw material along with its removal.
Therefore, it would be desirable to separate the carbon monoxide from such hydrogen chloride-containing waste gases, in order to prevent disadvantages caused thereby in a subsequent Deacon process, and simultaneously make use of the carbon monoxide in an economic manner.